Dry process molding of hard ferrite powders



May 23, 1961 J. C. VENERUS DRY PROCESS MOLDING OF HARD FERRITE POWDERS Filed June 4, 1959 INVENTOR. \/0JPH 6' Vila/ems h/Mym United States Patent DRY PROCESS MOLDING OF HARD FERRITE POWDERS Joseph C. Venerus, Plainfield, NJ., assignor to Steatite Research Corporation, Keasbey, N .J., a corporation of Delaware Filed June 4, 1959, Ser. No. 818,166

'1 Claim. (Cl. 18--'59.3)

This invention relates to a method and apparatus for dry feeding powdered, hard magnetic ferrites to molds and molding the same.

Among the objects of the invention is to provide a feeding and molding process for dry powdered hard ferro-magnetic powders to form the same to shaped bodies which produces products which are at least equivalent to the products formed by wet molding processes.

It is well-known that the quality of barium ferrite or similar hard ferrite magnets is improved when the powdered ferrites are pressed to shape in a magnetic field. The magentic field aligns the single-domain particles, thereby producing a magnetic anisotropy in the pressed pieces which remains after the sintering process. The magnetic anisotropy greatly increases the magnetic quality of the end product.

It is further known that pressing in a magnetic field can be done either wet or dry. At present, the wet-pressing method is principally used, because in a liquid suspension the particles are more easily aligned than when they are in the dry condition and therefore, the performance values of magnets made by wet pressing are higher than those of dry-pressed magnets. On the other hand, wet pressing is more expensive than dry pressing because the removal of the suspension liquid during pressing is time consuming.

The present invention is based on the discovery that a barium or hard ferrite powder with individual particle diameters below the single domain size (1.3 microns), for example, between 0.2 and 1 micron, and which have a tendency to agglomerate, can be loosened and made flowable by subjecting the powder to an A.C. field and can then be prealigned by a high field strength pulsating D.C. field. The A.C. field breaks up any agglomerates which have formed and prevents the formation of new agglomerates. The small grains are spontaneously magnetized providing single magnetic domains with a high coercive force. They, therefore, when subjected to an- A.C. field of normal line frequencies (50 or 60 cycles), rotate in synchronization with the field. It is necessary only that the torque moment coming from the A.C. field is stronger than the friction between the particles. This friction effect can be overcome by providing an A.C. field strength of about 1000-3000 oersteds and by pretreating an absolutely dry powder to loosen the same in a micronizer or a cage mill.

In the step of treating in the A.C. field, air which is entrapped by the powder particles becomes intensely agitated during the rotation of the particles; therefore, a large increase in volume takes place.

The term hard magnetic or ferromagnetic ferrite will be understood to mean a ferrite with a high coercivity and a high remanance. Examples of such ferrites are those of the formula MO.6Fe O wherein MO is a bivalent metal oxide of the group barium, strontium and lead oxides, and mixtures of such oxides.

of about 1500-3000 oersteds or more.

The pulsating D.C. field, applied to the mass of particles just before and while entering the mold, may have a strength of 10,000 oersteds for a fraction of a second, for example, and must have the same polarity as the constant D.C. field Within the die cavity.

The invention will be more clearly understood by reference to the following specific example thereof when considered in connection with the accompanying drawing in which:

The figure discloses an apparatus for carrying out the process of the invention.

The apparatus shown comprises a hopper 1 for holding a substantial supply of the powder 10 to be molded, and a powder conduit 2 which in this apparatus takes the form of a flexible pipe or tube. The lower end of flexible pipe 2 connects to a filling shoe 3. The hopper 1 has a coil 4 spirally wound about the same, the tube 2 has the coil 14 wound thereabout and the filling shoe 3 has a coil 24 wound thereabout. The innermost part of the coil surrounding the die case 5 constitutes a separate winding 24a and is connected to the same pulse generator as coil 24. Said coil 24a may or may not be utilized as compared to coil 24 which must be operated in any case. Suitable connections (not shown) supply an alternating current to coils 4 and 14. The said coils 4 and 14 may be connected together as one coil or may be separate coils, each supplied with 50-60 cycle alternating current, for example. Alternating currents of lower or higher frequencies may be employed but the 50 or 60 cycle current are readily available and are adequate.

To avoid eddy currents and other complications due to magnetic fields or other electrical phenomena the parts 1 and 2 are constructed of materials of high resistance. For example, both of these parts may be of plastics. The mold shoe 3 should be of non-magnetic material but can be made of metal if desired as long as the metal is non-magnetic.

The mold 6, as shown, comprises a die cavity 15 surrounded by the cylinder 16, the bottom of which is closed by plunger 17 which is movable axially in said cavity 15. A suitable male mold member, not shown, is provided to compress the powder introduced into the cavity 15 against plunger 17 after the mold cavity is filled with powder and after the mold is moved away from the filling station. The mold as shown can be fitted into the opening 18 of a table 20 along which the filling shoe may be slid. The table 20 is adapted to shield the coil 5 which surrounds the cylinder 16. Coil 5 is connectable by a suitable means (not shown), a switch, for example, to a source of direct current by suitable terminals not shown. The coil 5 may be constructed to produce when energized, a steady field with a strength The mold 6 may be made of conducting metals, such as copper, steel, etc.

in operation, the coils 4 and 14 are continuously supplied with alternating current so that powder 10 is free of agglomerates, is free flowing, and is somewhat expanded due to the air currents built up by the oscillating grains of powder. After the coil 24 has been pulsed once or several times with a 10,000 oersted D.C. field, for example, the filling shoe 3 is moved over the die cavity 15 whereupon the steady D.C. field of coil 5 draws the powder into the mold cavity 15 and the pulsed D.C. field in coil 24 and 24a promotes the alignment of the particles. The direction of the pulsating field of coil 24 and 24a is the same as that of coil 5. During the compression step which follows, the alignment of the particles becomes fixed.

Example Barium ferrite particles Which have been ground to a size of less than 1.3 microns and completely dried are fed to hopper 1 of an apparatus similar to that of the drawing. Alternating current is supplied to coils 4 and 14 as the powder passes to filling shoe 3. The pulsing field of coil 24 is applied to the powder prior to and during its passage into cavity 15. The powder is com pressed at a pressure of about 9000 pounds p.s.i. (pounds per square inch) and the pressed product is removed from mold 6 and fired at 1200 to 1280 C. Since no suspension liquid has to be removed during pressing the process is much faster than the Wet molding process.

The properties obtained by the process of the invention as compared with the properties of ferrites obtained by other processes are as follows:

Ferrite Energy Value 1. Barium ferrite prepared as above 2.7 to giglXlO gauss oers e s. 2. Barium ferrite prepared by dry filling and then 2.4 to 2.7 X10 gauss orienting inside the die case by a DD. field. oersteds. 3. Wet pressing and orienting by D .C. field 3.3 to 3.6360 gauss oerste s.

.4 fications will suggest to those skilled in the art many other modifications thereof. It is accordingly desired that the appended claim shall not be limited to any specific feature or details thereof.

I claim:

In the production of a molded and fired, hard magnetic body the steps comprising providing a finely divided dry, loose, powder of the hard magnetic ferrite to be molded, feeding said powder towards a mold cavity, subjecting said powder before it enters said mold cavity first to an alternating current field adapted to ro tate said magnetic particles in synchronization therewith, and then to a high strength pulsating D.C. field, transferring the powder to the mold cavity and simultaneously aligning the particles as they move into said mold cavity by applying a direct current field about said mold cavity oriented in the same direction as said pulsating D.C. field, thereafter compressing said powder while the powder particles are still aligned and firing the resultant product.

References Cited in the file of this patent UNITED STATES PATENTS 1,930,788 Buckner Oct. 17, 1933 2,064,773. Vogt Dec. 15, 1936 2,437,127 Richardson Mar. 2, 1948 2,742,185 Landry Apr. 17, 1956 2,873,048 Gear Feb. 10, 1959 FOREIGN PATENTS 558,366 Canada June 3, 1958 1,054,188 Germany Apr. 2, 1959 

